Process and apparatus for the concentration of an alkaline solution

Abstract

An alkaline solution film flowing from the top toward the bottom in falling-film evaporator tubes (1) is heated by a heat-transfer fluid flowing from the bottom toward the top in a narrow heating jacket (5). Since large temperature differences occur between the heat-transfer fluid and the alkaline solution film in the upper heated portion of the evaporator tube (1), in contrast to the lower portion, especially at high flow rate as it occurs in a narrow heating jacket (5), the heat flow density can at that location lead to a lack of wetting of the evaporator tube. To avoid such critical heat flow densities, the upper section (5a) of the heating jacket (5) is conically widened and thus the flow rate in this section is reduced to such an extent that a critical heat flow density is not reached at that point.

What is claimed is: 
     
       1. An apparatus for concentrating an alkaline solution which comprises: at least one evaporator tube,   means for introducing the alkaline solution to be concentrated into the upper portion of said evaporator tube so that a film of said alkaline solution falls along the internal surface of said evaporator tube,   a collecting chamber communicating with the lower portion of said evaporator tube for recovering the concentrated alkaline solution,   heat exchange jacket means surrounding and spaced apart from said evaporator tube along its substantial length, said jacket means and evaporator tube defining an annular channel therebetween for receiving a heat-exchange fluid, said annular channel being sized and dimensioned at the upper portion of the evaporator tube for reducing the flow rate of the heat exchange fluid to such an extent that a critical heat flow density is not reached and sufficient to avoid the Leidenfrost effect, and   means for introducing the heat exchange fluid into the lower portion of said heating jacket and means for removing the heat exchange fluid from the enlarged upper portion of said heat jacket.   
     
     
       2. The apparatus of claim 1 wherein the heat exchange jacket is enlarged cylindrically or conically to form said annular channel. 
     
     
       3. The apparatus of claims 1 and 2 wherein the heat exchange jacket having the same dimensions as in the lower portion thereof extends into the enlarged annular channel and aperture are provided in said extended portion to permit egress of said heat exchange fluid through said extended portion into said enlarged annular channel. 
     
     
       4. The apparatus of claim 3 wherein the inner surface of the perforated jacket is uniformly provided with a plurality of nubs for ensuring a uniform distribution of the heat-transfer fluid around the evaporator tube. 
     
     
       5. The apparatus of claim 1 wherein the means for introducing the alkaline solution to be concentrated comprises a distributor container and conduit means for introducing said alkaline solution into said distributor container, the upper end portion of said evaporator tube extending above the bottom of said distributor container whereby the alkaline solution introduced thereinto is permitted to fall as a film over the end portion of said evaporator tube. 
     
     
       6. The apparatus of claim 5 wherein the top of the evaporator tube is serrated. 
     
     
       7. The apparatus of claim 1 wherein the collecting chamber is provided with a liquid outlet and a vapor outlet.